All-ZnO light-emitting diodes (LEDs) offer the promise of a low-cost, brighter alternative to existing GaN-based light emitters for solid-state lighting applications, in part due to the higher exciton binding energy of ZnO. We have used ISE TCAD (TM) simulations to examine the effect of active, n- and p-layer dopings and thicknesses on the optical output intensity and current-voltage characteristics of both vertical and lateral geometry ZnO LEDs. The latter geometry is attractive for ultralow structures with the ZnO deposited on glass substrates. The current density distribution is more uniform in the vertical structures but there is little difference in optical output power as a function of doping or layer thickness between the two geometries. (c) 2006 American Vacuum Society.